Multifilament coated conductors are intrinsically inferior to monofilament coated conductors from the viewpoint of the robustness against quench / thermal runaway. Quench detection and protection conditions of monofilament coated conductors have been studied using short straight sample. In this study, we compared the maximum hot-spot temperature during quench detection and protection procedure of multifilament coated conductors with that of monofilament coated conductors at the same operating current (total current density ~ 400 A/mm2) under various quench detection and protection conditions (detection voltage, time constant of current decay, etc.).
Lengths of coated conductor samples are 230 mm (effective length between current terminals = 180 mm). Multiple voltage taps were attached on the samples by soldering. Quenches were initiated by using a small resistive heater at center of the samples. An FPGA module was used to monitor the voltage of entire sample and control power supply output, which enabled us to simulate quench detection and current shut down.
In quench experiments, the samples were cooled to 30 K by using a GM cryocooler. The magnetic field up to 2 T could be applied, and the sample current up to 500 A could be supplied. Quench detection and protection were simulated by the following 3 steps. First, quench was detected by the voltage of entire sample if it reached a threshold voltage. Next, a period of time was waited to simulate the time for activating circuit breaker. Finally, current was decreased exponentially, which simulated magnet energy dump using external resistor. Before and after each quench experiment, the critical currents of the sample were measured to see whether the sample has been degraded or not. Hot spot temperatures were calculated using the current sharing model and the temperature dependence of resistance of copper stabilizer.
Acknowledgments:
This work was supported by JST-Mirai Program Grant Number JPMJMI19E1, Japan.